Benefits of Microwave Reactions

As a microwave chemist, I hear a lot of questions from scientists that are new to this field of synthetic chemistry. Here are a few commonly asked questions and topics of interest or discussion.

How do microwaves heat matter?

Microwaves are electromagnetic waves, consisting of electric and magnetic field components. When microwave energy passes through matter, the material’s dipole field attempts to realign itself with the oscillating electric field of the microwave. The molecules in the material never have enough time to exactly follow the oscillating field, so the continual re-orientation of the molecules results in energy loss through molecular friction and dielectric loss. The amount of heat generated by this process is dependent on the ability of the molecules to align themselves with the frequency of the electric field. This is known as the microwave dielectric heating effect. Microwaves operate at a frequency of 2.45 GHz, which is below the energy needed to break a bond, but high enough that a molecule with a dipole releases heat through the rotational motion of the dielectric heating effect.

How quickly can my reaction take place in a microwave reactor?

Microwave reactors are equipped with pressure sealing technology, which means heating conditions above reflux are often used in microwaves. The speed at which it can reduce reaction times can be found by applying the Arrhenius law/equation:

Arrhenius law/equation

For every increase of 10° C, the reaction time is cut in half. For example, a reaction that may take a full day at room temperature would take approximately 2 minutes at 120° C. Many microwave reactors, including CEM’s Discover line, have the capability to heat to 300° C and have a maximum pressure threshold of 435 psi.

How do microwave reactors compare to traditional heating mantles?

The dielectric heating mechanism of microwaves is well known to be an efficient source of volumetric heating. Traditional heating methods (heating mantles, etc.) transfer heat to the wall of the reaction vessel, which eventually gets dispersed to the reaction solution. This is a slow and inefficient process that can create gradients of temperature. These can lead to unwanted side reactions. Microwaves, however, directly apply energy to molecules, and not to the reaction vessel, being able to volumetrically heat the mixture to the desired temperature within minutes. This often leads to higher yields of reaction products when using microwave-assisted chemistry.

What if I want to scale up reactions?

There are a multitude of microwave techniques that can be used for scaling up reactions. The MARS 6 system can perform high temperature reactions (300° C) with high pressures (800 psi) in up to a 1.2 L sealed volume or at reflux temperatures and pressures in up to a 5 L round bottom flask. The Autosampler 12 or Autosampler 48 robot upgrades for the Discover 2.0 can also be easily used for scale up needs, sequentially processing 35 mL pressure vessels. Flow cells can also be used to increase reaction throughput in a continuous processing format; CEM offers both 10 and 80 mL continuous flow vessels.

Can I easily adjust temperature and pressure settings?

CEM’s Discover line boasts the flexibility to control more heating types than any other microwave synthesizer. Among these, one can heat to a target temperature quickly and consistently, or explore the nature of microwave heating with advanced pressure and temperature control programming.

Can I use gaseous reagents?

Chemistry using gaseous reagents, such as hydrogenations and carbonylations can easily be performed using the Gas Addition Kit. This can also be used to ensure an inert atmosphere during microwave irradiation.

Is it feasible to do low-temperature reactions?

Microwave reactors can be beneficial to accelerate reactions at temperatures as low as -80° C with CEM’s Discover CoolMate. The CoolMate utilizes the fact that microwave energy is transferred directly to molecules in solution rather than through a vessel wall. This sets it up as an incredible tool to accelerate reactions that are maintained at low temperatures. The CoolMate’s vessel and cooling technology keep the bulk temperatures of the reactants low, while also providing microwave transparency to insure full energy transmission to the reactants. This can be useful in temperature-sensitive chemistries such as lithiations or carbohydrate synthesis.

How can microwaves incorporate green chemistry principles in my lab?

Chemistry using microwave reactors follow many of the 12 principles of green chemistry, provided by the ACS Green Chemistry Institute.

Here are a few examples:

Efficient heat transfer leading to atom economy
Conventional heating is a superficial heating process; the energy is transferred from the surface to the bulk by convection and conduction pathways. In contrast to this fairly inefficient mode of heating, microwave irradiation provides direct heating to the bulk reaction mixture, providing an effective heating strategy. This often leads to near quantitative yields, maximizing the atom economy.

Safer solvent usage and the prevention of waste
Reactions in water can occur readily in a microwave reactor due to the ability to easily perform reactions near the supercritical temperature. The dielectric constant of water changes from 78.5 to 27.5 at 250° C, meaning that water can be considered as a pseudo-organic solvent. Water has been shown, when combined with microwave heating, to be an excellent solvent to use instead of DCM or toluene. Oftentimes, the products crystallize out after the reaction takes place and the mixture cools down, simplifying the purification procedure, and reducing waste when compared with other separation methods.

Catalysis
It is often possible to reduce the amount of catalyst needed for a chemical reaction when using microwave reactors. Transition metal catalysts show high activity under microwave irradiation conditions. This is due to the efficient interaction of the metal complexes with the microwaves, enabling the catalyst to heat to very high temperatures which can overcome many activation barriers to reactions.
References Antonio de la Hoz, Angel Díaz-Ortiz and Pilar Prieto, CHAPTER 1: Microwave-Assisted Green Organic Synthesis, in Alternative Energy Sources for Green Chemistry, 2016, pp. 1-33 DOI: 10.1039/9781782623632-00001.

McGowan, Cynthis B. and Leadbeater, Nicholas E. Clean, Fast Organic Chemistry: Microwave-Assisted Laboratory Experiments. 2006. CEM Pub., Matthews, NC.

 

Instruments

Discover 2.0 Microwave Synthesis System

Discover 2.0


Microwave Synthesizer


Create new molecules and compounds with unrivaled reaction accuracy, safety, and flexibility.

Discover Microwave Synthesis System

Discover


Microwave Synthesizer


Explore new chemical space, unattainable by conventional sealed vessel reactions.

MARS 6 Synthesis - Parallel Scale-Up Microwave Synthesizer

MARS 6 Synthesis


Parallel, Scale Up Microwave Synthesizer


Parallel microwave synthesis for teaching and industrial laboratories.